Recently, optical networks are realized in various forms from a highway-based network connecting large cities to an access-based network connecting a base station and ordinary houses. A metro-network that mainly connects base stations employs a Wavelength Division Multiplexing (WDM) technique and optical hub apparatuses such as an Optical Add/Drop Multiplexer (OADM) apparatus and Wavelength CROSS Connect (WXC) that realize insert/branching and path switching of an optical signal in a wavelength.
Thereby, in a two-core bilateral optical fiber network, a large-capacity network is constructed or networks of various connection forms such as ring connection and mesh connection are constructed. An apparatus such as that conforming to a standard such as Synchronous Optical NETwork/Synchronous. Digital Hierarchy (SONET/SDH, hereinafter, “SONET”) is employed as a specific communication transmitting apparatus.
In an access network, a network of a star topology has been realized using a one-core bilateral transmission path where a terminal point node of a metro network is used as the base point. A star topology network employs, for example, a Passive Optical Network (PON) system technique. Recently, the band for a PON system is drastically increasing (for example, G-PON and 10GE-PON) and further improvement of the functions thereof is expected.
In a PON-scheme optical communication system that is a typical one-core bilateral system, a plurality of subscriber terminals (ONUs: Optical Network Units) are present for one local station (OLT: Optical Line Terminal) and these two parties communicate using one-core optical fibers. For example, an UpLink (ONU to OLT) optical signal is allocated with a wavelength of 1310 nm and is transmitted in a burst mode. A DownLink (OLT to ONU) optical signal is allocated with a wavelength of 1490 nm and is transmitted in a continuous mode.
As above, it is general that a metro network employs a path transport network such as the SONET, and an access network employs a PON system. However, considering the metro network and the access network from the viewpoint of integration, the existing network configuration is not regarded as the optimal network configuration in respect of its price and efficiency.
For example, a SONET-scheme transmitting apparatus is an expensive and transmission-path-connection-based system, whereas expansion of the bandwidth and reduction of the cost are rapidly proceeding for the PON system. Therefore, when the PON system is also applied to, for example, the metro network, realization of a lower-cost and higher-efficiency network than an existing optical network can be expected. For example, a network using a SONET frame in a portion of a PON system is disclosed (see, e.g., Japanese Patent Application Laid-Open Publication No. 2006-237769).
However, a SONET-scheme optical communication system that forms a metro network uses two-core bilateral transmission paths while a PON-scheme optical communication system that forms an access network uses one-core bilateral transmission paths. Therefore, it is difficult to realize a PON-scheme optical communication system in a SONET-scheme optical communication system.
Therefore, a problem has arisen that the network cannot be flexibly designed. More specifically, because an amplifier or an isolator is provided on each node in a two-core bilateral transmission path in a SONET-scheme optical communication system, each core of the two-core bilateral transmission path can pass an optical signal only in one direction. Therefore, no PON-scheme optical communication system can be realized by bilaterally using one core of the two-core bilateral transmission path.